Individual crystals in the microstructure of a solidified metal alloy are referred to as grains. Grain refinement is a very important technique for improving the mechanical properties of metallic components because grain size affects grain boundary strengthening. It is also well known that finer grain structure in direct-chill cast alloy billets can increase the maximum extrusion speed and hence reduce the cost of extruded products. Furthermore, finer grain sizes will improve the quality of alloy billets and castings due to a reduction in size of defects such as hot cracking and microporosity. Grain refining can also result in finer and more uniform distribution of intermetallic particles in the as-cast material which will increase the efficiency of subsequent homogenizing treatment and reduce heat treating time. The effect of grain refining is particularly significant in magnesium alloys.
Aluminum, manganese, and zinc are common alloying elements for magnesium-based alloys. Aluminum in amounts up to about ten percent by weight is used in many magnesium alloys. Many grain-refining techniques have been developed for melts of Mg—Al alloy systems, such as superheating, carbon addition, agitation, and additions of particles such as Al4C3, AIN, SiC, TiC, CaC2 and solute elements such as B, C, Ce, La, Nd, Sr, and Y. Among these techniques, addition of carbon-containing agents to the melt offers more practical advantages because they require lower operating temperatures and their grain-refining effect is slower to fade. However, adding carbon-containing agents such as C2Cl6 or CCl4 have environmental concerns due to the emission of chlorine. Also, grain refiners like C2Cl6 are not very effective in heavy solidified sections.
The commercial magnesium alloy, AZ31 (Mg-3% Al-1% Zn, concentrations in weight percent) offers a good combination of mechanical properties, castability, and extrudability for automotive vehicle components and other articles of manufacture. But there remains a need for an improved method of treating melts of such aluminum-containing magnesium-base alloys to control and reduce the grain size in the solidified product.